The present invention relates to a fixing device for melting and fixing a toner image on a transfer material in an image forming apparatus and to an image forming apparatus using the same.
The present invention is more particularly related to a heating device comprising a heating medium of electroconductive member, magnetic field generating means for generating induced current in said heating medium and a voltage source for actuating the magnetic field generating means, wherein a developer is fixed on the recording material by induction heating of the heating medium, and to an image forming apparatus using the same.
The image forming apparatus of an electrophotographic type normally comprises a fixing device which fuses and fixes toner which comprises resin material, magnetic member, coloring material or the like and which is electrostatically attracted on a transfer material by nipping and feeding such a transfer material through a nip formed between rotating heating means (roller, endless belt member or the like) and pressing weans (roller, endless bolt member or the like).
In one type of such fixing devices, the heating means comprises an excitation coil and an electroconductive layer, wherein a magnetic flux generated by the excitation coil produces eddy current in the electroconductive layer provided inside the fixing roller (fixing member), by which heat is generated by joule heat, as disclosed in Japanese Laid-Open Utility Model Application Sho 51-109736. With this method, the heat generating source can be disposed very closely to the toner, and therefore, the time required for the temperature of the surface of the fixing roller to reach the proper fixing temperature upon the start of the fixing device can be shortened as compared with a conventional heating roller type using a halogen lamp. In addition, the heat transfer path from the heat generating source to the toner is short and simple, and therefore, the heat efficiency is high.
As regards a safety apparatus for the fixing device, a temperature fuse or a thermostat is mounted. Conventionally, the temperature fuse and/or the thermostat are directly contacted, but doing so damages the surface of the fixing member with the result of shortened service life. For this reason, non-contact mounting is desired. There is a method in which they are disposed out of contact from the fixing member, and the temperature is detected thereby. With this method, the temperature sensing response is slow in consideration of speedy temperature rise provided by the self-heat-generation of the fixing member. To obviate this problem, selection of a high precision temperature fuse, thermostat or the like is difficult. It is desirable that temperature of a portion outside the fixing member is detected by increase and decrease of the current flowing through the coil, so that abnormality such as excessive temperature rise of the fixing member is detected. Japanese Patent Application Hei 9 1978521 discloses a coil in the fixing roller is projected to the outside, and the projection is disposed so as to be influenced by the magnetic field generated by a coil.
However, with this structure, the ambient temperature around the temperature detection member becomes very high because of the heat radiation from the fixing member and the heat generation of the coil per se. Therefore, there arises a problem that excessive temperature rise is discriminated even when the temperature of the fixing member is lower than the limit temperature since the temperature detection member detects the temperature of the fixing member added with the temperature provided by the heat generation of the coil per se.
Accordingly, it is a principal object of the present Invention to provide a fixing device and an image forming apparatus wherein a coil and a heat generating element having a small heat generation, so that excessive temperature rise of the fixing member is detected correctly.
According to an aspect of the present invention, there is provided a fixing device comprising a first coil for generating a magnetic field; a first heating medium for fixing an unfixed toner image on a recording material by heat, said first heating medium having an electroconductive layer which generates heat by eddy current produced by the magnetic field formed by the current through the first coil; a second coil, electrically connected with said first coil, for generating a magnetic field; a second heating medium having an electroconductive layer which generates heat by eddy current produced by the magnetic field formed by the current through the first coil, wherein a temperature of said second heating medium is lower than that of a temperature of said first heating medium in operation.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.